A typical wireless communication system includes one or more base stations, each radiating to define one or more cells in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices (whether or not user-operated) can operate. Further, each base station of the system may then be coupled or communicatively linked with network infrastructure such as a switch and/or a gateway that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the system may thus engage in air interface communication with a base station and thereby communicate via the base station with various remote network entities or with other UEs served by the system.
In general, a wireless communication system may operate in accordance with a particular air interface protocol or radio access technology (RAT), with communications from a base station to UEs defining a downlink or forward link and communications from the UEs to the base station defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Long Term Evolution (LTE) (using Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communication (GSM), WI-FI, and BLUETOOTH. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between cells, and other functions related to air interface communication.
In accordance with the air interface protocol, each of the one or more cells of such a system may operate on a respective carrier, which could be frequency division duplex (FDD) or time division duplex (TDD), defining one or more frequency channels each typically within an industry standard frequency band. An FDD carrier defines separate frequency channels for downlink and uplink communication, so that UEs can engage in downlink and uplink communication. Whereas, a TDD carrier defines a frequency channel multiplexed over time (e.g., on a recurring basis) between downlink and uplink use, so that UEs can engage in downlink communication in only particular time intervals and uplink communication in only other time intervals.
Each carrier in such a system may also define a number of sub-channels for carrying information between the base station and UEs. For instance, each carrier may define a pilot channel, reference channel or other resource on which the base station can broadcast a pilot signal, reference signal, or the like that UEs may detect as an indication of coverage and may measure to evaluate coverage. Further, each carrier may define a downlink control channel for carrying scheduling directives and other control signaling from the base station to UEs, and an uplink control channel for carrying service requests and other control signaling from UEs to the base station, and each carrier may define downlink and uplink traffic channels or the like for carrying bearer traffic between the base station and UEs.
When a UE initially enters into coverage of a wireless communication system (e.g., powers on in coverage of the system), the UE may scan for a strongest (e.g., highest signal-to-noise ratio) reference signal, defining a strongest cell. Further, the UE may read system information broadcast from the base station in that cell and may engage in a process to register itself to be served by the base station's cell and generally by the system.
In an example process, for instance, the UE and base station may first engage in signaling to establish a radio-link-layer connection between the UE and the base station, the UE may then transmit an attach, registration, or association request message on an uplink control channel of the cell, and the base station and/or supporting infrastructure may then authenticate and authorize the UE for service, establish a record indicating where in the system the UE is operating, establish local profile or context records for the UE, and provide an attach accept message to the UE. The UE may then be served by the base station's cell in an idle mode or a connected mode.
Further, while a UE is served by a base station's cell, the UE may also monitor the signal strength of that cell and of other cells in an effort to ensure that the UE continues to operate in a best (e.g., strongest) cell. If the UE detects that its serving cell becomes threshold weak and/or that another cell is threshold strong (e.g., threshold stronger than the serving cell), then the UE may hand over from being served by its current cell to being served by the other cell. For instance, the UE may transmit to its serving base station a measurement report indicating the other detected cell, and the base station and/or supporting network infrastructure may responsively process handover of the UE from the serving cell to the other cell. Alternatively, the UE may engage in idle handover by itself to the other cell and possibly transmit a location area update or tracking area update to the network to notify the network where the UE is now operating.